Systems

Hot Water & Heating

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Introduction

Hot water and heating combined are responsible for 41% of a household's greenhouse gas (GHG) production. On an annual basis, an average Canadian house produces 2.5 tonnes of GHG. Thus, approximately one tonne of GHG is a direct result of hot water and heat consumption.

HomeAlive uses a tried-and-true evacuated-tube system for heating food-grade glycol that, in turn, heats the domestic water (for showers, washing machine and dishwasher) and the whole house too. Except for in the deepest of winter, it provides the entire needs for the house.

It's a system that automatically reduces the GHG production by one tonne.

How it Works

The solar hot water heating system at Home Alive uses a closed loop design. This means that solar heat captured by the collectors is transmitted to water in a storage tank with a heat exchanger. There is no direct movement of the stored water into the collectors.

The four, roof-mounted evacuated-tube solar collectors have a pump that moves food-grade glycol through a pipe that is in contact with the collector's hottest surface. The pump starts automatically whenever the collector temperature sensor is at least 20 deg C warmer than the current water temperature in the hot water storage tank, indicating that it can effectively transfer its collected heat. Even in partial sun and on some overcast days, these collectors generate glycol temperatures of around 60 deg C. The warmer that the collectors get relative to the storage tank temperature, the faster the pump runs in order to take maximum advantage of the sun's output.

Capacity

At its peak, this system will provide a flow of 2.5 litres per minute, or about 1/6 of what most conventional residential hot water systems can output. However, the solar collectors generate this heat any time there is sunlight and then store it in the tank so that we have hot water when we need it. In fact, because evacuated solar hot water collectors surround the copper absorption surface with a glass tube that is in a vacuum and because a vacuum is a perfect insulator, outdoor air temperature does not matter. Evacuated tube type collectors generate heat in all climates, all four seasons and will even produce some heat from UVA and UVB non-visible solar radiation that penetrates clouds on overcast days.

The only time when this system cannot meet the hot water and heating needs is in the deepest winter months of December and January, when the combination of extreme cold and low number of daylight hours. Otherwise, for ten months of the year, it is sufficient to provide heat for the house's hydronic heating system and for all domestic hot water needs (ie showers, faucets, dishwasher).

Efficient Backup System

During cold days, or after consecutive cloudy days or perhaps during times of high domestic use (ie repeated showers) the solar hot water system is insufficient to handle all needs. For this reason an instantaneous (or tankless) hot water heater is used as a failsafe backup.

Instantaneous heaters are the norm in Western Europe and other parts of the world but are less common in Canada. Their significant advantage over a typical hot water storage tank with a built-in heater is that they only operate when there is a need for hot water whereas a typical tank is always heating water whether you need it or not (ie during work hours, when we are asleep or on holiday). Thus, instantaneous heaters eliminate what is known as "stand by" energy losses, energy used to heat water that isn't required.

Backup Control Strategy

In this application, the instantaneous heater is activated when its dedicated heater pump turns on. The heater pump in turn is activated when a combination of conditions becomes true:

1. the hot water storage tank temperature in the top third of the tank (the hottest part since heat rises) drops below a set temperature (often 120 degF)
2. the air temperature is colder than the residents' thermostat setting
3. the slab temperature is colder than a preset value (somewhere between 24 and 28 deg C). Since hydronic heating takes time to heat up, this sensor prevents the unintentional overheating of the house by limiting the amount of new heat going into the slab even if the desired thermostat temperature has not yet been met

In the near future, Home Alive's automated computerized control system will add more conditions to the starting of the back up heater.

1. the control system will track outdoor air temperature, relative humidity and lumen (light) level while simultaneously tracking indoor air temperature. It will then establish a history of how outdoor conditions impact on indoor conditions. For example, especially given Home Alive's many south-facing windows, simple passive solar heat (sunlight entering through window glass and heating the interior air and contents) can warm the house when the sun is out even during cold winter days. Knowing how fast the house heats up based on how warm, humid and bright it is outside, allows the control system to anticipate the passive (free!) heat gain and turn down the heating system even before the air thermostat temperature setting has been exceeded. This strategy is used in many commercial buildings and saves valuable energy while providing a more consistent indoor temperature and improving resident comfort level
2. the control system will establish a history of use of heat and hot water and prevent the backup heater from coming during traditional periods of non-use. This will allow the house to naturally cool in the evening when the residents are sleeping or perhaps in the day while they are at work. At the same time, the control system will try and time operation of appliances such as dishwasher and clotheswasher to take advantage of excess hot water being produced by the solar hot water system so that the backup heater is not required for these same activities after the sun has disappeared.

Hydronic (or Radiant) Heating

Hydronic (also commonly known as "radiant") heating systems are gaining acceptance as a high-comfort and extremely efficient means of distributing heat throughout a house.

In Home Alive hot water is drawn by a pump from the Hot Water Storage Tank and sent through four different loops of 1/2" pipe buried at about the midpoint of the 4" thick concrete slab floor. This pump is activated by the same control strategies listed above.

Because the heating water is drawn directly from the Storage Tank and not transferred indirectly through a heat exchanger, this part of the system is known as "open loop". In an open loop system, the same water that passes through the pipes in the floor could very possibly next come out the kitchen faucet or the bathroom shower! Very little energy is lost or "wasted" since there is one level of heat exchange is eliminated.

Also because the stored hot water is often maintained at 120&176; F, and the water need only enter the floor pipes at 90° F in order to ideally heat the house, a mixing valve is required to ensure this lower temperature is not exceeded. This Three Way Mixing Valve takes blends some of the now slightly cooler return water from the floor and adds it to the fresh hot water to arrive at the desired consistent temperature for the Supply to the floor. The remaining cooled water is returned to the bottom of the Hot Water Storage Tank to be again heated by the solar hot water or backup heater and the process continues.

Good for You, Good for Environment

The benefits of this system are many. There are obvious, GHG reductions and the long-term cost efficiencies to the owner. The energy source is essentially free and renewable. Although it appears to have complex components, the maintenance is relatively simple. It is no more complex than any other furnace or forced-air system. The cost and availablity are stable, and not dependant on third-party suppliers, such as with utilities of electricity, gas or oil. As well, it contributes to a better air quality indoors, as the heating is gentle and does not dry the air out (as does electricity) or send dust into the air (as does forced air).

For More on Solar and Hydronic Systems

• Learn about the HomeAlive systems hands-on. Everdale offers seasonal workshops on different aspects of the systems. You can also take part in guided Saturday Tours of HomeAlive. Visit www.everdale.org
• Check out the HomeAlive Buyer's Guide for suppliers and equipment of these systems.

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